There was usual slap and tickle, so I wasn't paying attention otherwise I would appear impolite. Suddenly I realised that it was THE slap and tickle. The guy was giving her quickies - constantly going for her ribs, stomach, back, armpits and even sides of the breasts. Now and than she broke into sweet giggles. Because of the breast tickles she became embarassed and asked him to stop WHILE IN THE UNDERGROUND CART. He stopped and I thought that if the guy had balls, he would start some serious tickling OUTSIDE. I decided to follow and see. Fortunately for me they got out at my station. Using usual tricks I positioned myself about ten meters behind them. I was right ... Perhaps ten meters they walked hand in hand, then there was a narrow pavement so the boy "naturally" placed himself behind her. After ten second or so he started serious tickling assault on her ribs, armpits and tummy. She was laughing, wriggling and staggering forward to escape him but he followed her every move. They struggled for about twenty meters, then the girl was laughing so hard that she couldn't walk anymore. Pretending I am just going the same way (which was true, my home was nearby) I walked by them, hearing her laughter and begging for mercy. Of course I couldn't stand and watch so I kept walking, all the time listening to her hysterics. In one point she became really angry with him, screaming "I can't stand it!" but in a very angry voice. (She managed that despite laughing hysterically). It was clear that wasn't fun for her anymore, but the guy was so captivated by the action (who could blame him), that he missed the altitude. I pretended to throw some garbage to the nearby thrashcan so I was able to take a glimpse back. She broke free from his grasp, sat on the ground and refused to go with him any further. I couldn't look more, so I went home. There I left my bag, caught the garbage and hurried out to see how the situation developed, but they already weren't there.